126 



THE THEORY OF EVOLUTION 



FIG. 27. a, Ripe pollen grain of 

 an Angiosperm ; b, ' germinat- 

 ing ' pollen grain ; g, generating 

 germ ; v, vegetative germ. 



formation is transformed either into fruit or pollen- 

 bearing leaves, or into protective or attractive means in 



connection with the fertiliza- 

 tion. ' Seed plants ' they are 

 called because it is through 

 the seed that the formation of 

 new individuals, separated 

 from the mother plants, is 

 effected. A seed is a multi- 

 cellular body, which, when it 

 leaves the mother plant, is already differentiated (a 

 grain of wheat, an apple pip). 



It has been possible to establish 

 the existence of an alternation of 

 generations in the Gymnosperms 

 and Angiosperms, but certainly of a 

 very debased kind. A series of in- 

 dications point to the pollen grains, 

 and the so-called embryo sac in the 

 bud germ, as being ' spores/ since 

 they, in the first place, without fer- 

 tilization, form a sort of prothallus 

 (second individual ?) in which first 

 arises the fertilizing cell proper 

 (sexual cell or, better, sexual germ). 

 The prothallus possesses, it is true, 

 in some Gymnosperms only three 

 small cells, or even only one in the Angiosperms the so- 

 called vegetative cell (Fig. 27). The macrospores produce, 

 on the other hand, in the Gymnosperms a particular 



FIG. 28. SECTION 

 THROUGH THE EM- 

 BRYO SEED OF A 

 GYMNOSPERM (Picea 

 vulgar is], 



E, ovum ; e, embryo 

 sac (Endosperm) = 

 prothallus ; p, two 

 pollen grains which 

 send tubes down to 

 the embryo ova. 



